1. Field of the Invention
This invention relates to semi-insulating zinc oxide (ZnO) single crystals. Such crystals can be used as substrates upon which electronic, electro-optic, and/or opto-electronic devices and circuits can be formed.
2. Description of the Related Art
Compound semiconductor single crystals are being used to fabricate various devices such as high electron mobility (HEMT) devices, field effect transistors (FETs), and light emitting diodes (LEDs). These devices are manufactured by depositing several layers of various materials on an initial substrate. Examples of commonly used compound semiconductor substrates are indium phosphide (InP), gallium arsenide (GaAs), and silicon carbide (SiC). Recently, there has been a spike of interest in zinc oxide (ZnO) single crystals for use as a substrate due to its wide band gap (3.3 eV), its stability at device operating temperatures (exciton binding energy of 60 MeV), and its close lattice spacing with gallium nitride (GaN) (there is only a 3% mismatch in lattice spacing between ZnO and GaN).
A semi-insulating substrate is preferred for device fabrication to effectively limit leakage current, thereby isolating individual components of the deposited device. The resistivity value of a semi-insulating material has come into question, and the general definition set forth by Carter et al. U.S. Pat. No. 6,218,680 will be followed, where a semi-insulating material must meet the minimum requirement of 1.5×103 Ω-cm at room temperature. Johnston et al. U.S. Pat. No. 6,211,539 have claimed semi-insulating behavior in InP for resistivity values as high as 1×109 Ω-cm, so this value will be accepted as the maximum semi-insulator resistivity value. It is of interest to note that Bylsma et al., U.S. Pat. No. 4,77,146, claim that resistivity less than 10×106 Ω-cm will exhibit excessive leakage current for GaAs, whereas Carter et al. similarly claim that for device isolation in SiC the resistivity value must be at least 533 104 Ω-cm. This reveals the variation in material electrical properties that will achieve true device isolation, where parasitic currents are avoided.
It has been found that ZnO exhibits semi-insulating behavior when a dopant is introduced into the crystal structure. Liu et al. grew Li doped ZnO films with resistivities ranging from 106 Ω-cm to 1011 Ω-cm. The applicant, Cermet, Inc., has grown Li-doped ZnO crystal with a measured resistivity of 2×108 Ω-cm. Nikitenko describes ZnO single crystals with Li additions yielding a resistivity of 104 Ω-cm. Liu et al. also grew N-doped ZnO films measuring 106 Ω-cm, while Joseph et al. grew 105 Ω-cm N-doped films. Lee et al. grew highly resistive (1010 Ω-cm). Cu-doped films, and Han et al. produced Mn-doped pellets of ZnO with resistivity of 108 Ω-cm, whereas undoped ZnO pellets yield a resistivity of 10 Ω-cm.